This invention relates to the reduction of vibrations in engine balance shaft drives. This invention has particular application to engine timing and balance drive systems driven by sprockets located on a rotating crankshaft. More particularly, the invention relates to a torsionally compliant sprocket for absorbing torsional loads and vibrations in a balance shaft drive system originating from a driving shaft.
Engine timing systems typically include an endless chain wrapped around a driving sprocket on an engine crankshaft and a driven sprocket on an engine camshaft. The rotation of the crankshaft causes the rotation of the camshaft through the endless chain system. A separate sprocket mounted upon the crankshaft may be used to drive a balance shaft system using a separate endless chain.
More complicated engine timing systems connect the crankshaft with two or more shafts by a pair of chains. The crankshaft includes two sprockets. Each chain is connected to one or more driven sprockets, including sprockets on each of the two overhead camshafts. Typically, the chain systems in more complicated engine timing systems will include tensioners on the slack side of each chain to maintain chain tension and snubbers on the tight side of each chain to control chain movement during operation.
Some engine timing systems have two (or dual) overhead camshafts for each bank of cylinders. The dual camshafts on a single bank can both be rotated by connection to the same chain. Alternatively, the second camshaft can be rotated by an additional camshaft-to-camshaft chain drive. The cam-to-cam drive chain can also include single or dual tensioners for chain control.
In some engine timing systems, especially those having a nonconventional firing order for the cylinders and those used in diesel engines, balance shafts are employed to counterbalance engine vibration. The balance shafts are driven by a chain connection from the crankshaft. Optionally, the balance shaft drive system may be utilized to operate an auxiliary drive such as a compressor or the like. The balance shafts are driven by the crankshaft and as a consequence, torsional vibrations and oscillations along the crankshaft may be transferred to the balance shafts and likewise throughout the timing system. Since balance shafts employ rotating weights, they are rotating systems having a high mass and inertia.
The rotating crankshaft may undergo resonance at certain frequencies. Since the balance shafts are coupled to the crankshaft by the balance shaft chain, the balance shafts are directly exposed to these extreme resonant torsional vibrations. The vibrations can cause high loads on the chain system for the balance shaft drive because of the high inertia of the balance shafts. Vibrations from the resonance of the crankshaft are often transferred throughout the system, including the balance shafts and associated engine elements and can significantly increase the load on the systems and components, increase the noise from the engine and increase wear and accelerate fatigue of the chains.
Some prior art timing systems use various damping devices to address the problem of vibrations. One example of such a prior art system uses a rubber damper piece which is placed against a sprocket and bolted to the shaft to absorb vibrations. However, the rubber damper piece may fracture from the extreme resonance vibrations. Other timing systems employ a weight that is positioned on the shaft and held against the sprocket by a Belleville washer. Frictional material is also placed at the area of contact between the sprocket and the weight. These systems, while effective at damping vibrations have drawbacks in terms of production, assembly and durability.
By contrast, the present invention focuses on absorbing the torsional vibrations of a crankshaft using a torsionally compliant sprocket. The torsionally compliant sprocket reduces the transfer of such vibrations and torque spikes to other parts of the engine system. The torsionally compliant sprocket interposes a resilient elastomeric layer between a hub portion and a sprocket rim portion of a driving sprocket mounted on the crankshaft to absorb vibrations and reduce transfer of the crankshaft vibrations.